Onto a recordable or rewritable optical disc such as a DVD-R (DVD-Recordable) or a DVD-RW (DVD-Rerecordable), information is recorded thereon by irradiating a laser light on a recording surface of the disc. At the areas on the recording surface of the optical disc where the laser light is irradiated, the property of the optical recording medium forming the optical disc is physically changed because of the increased temperature. This produces recording marks on the recording surface.
Namely, the laser light is modulated by recording pulses having time widths corresponding to information to be recorded, so that the laser pulses having lengths corresponding to information to be recorded are generated and irradiated on the optical disc. Thus, recording marks having lengths corresponding to the information to be recorded can be formed on the optical disc.
One approach recently used is control of a laser power to form a recording mark by a pulse train portion having a plurality of short pulses (referred to as “pulse train”), rather than by a single laser pulse. This approach, called “write strategy”, introduces less heat accumulation on the recording surface of the optical disc in comparison with the approach irradiating a single recording laser pulse. Therefore, uniform temperature distribution can be achieved on the recording surface on which the recording marks are formed. This can prevent undesired teardrop-shaped recording marks from being formed, and enables the formation of the recording marks of preferred shape.
In the above-mentioned write strategy, there is no problem at the time of normal-speed recording. However, since a clock becomes high speed at the time of high speed recording, control of the recording pulse for driving the recording laser becomes difficult. In this view, there is proposed a technique of using a recording pulse waveform including a top pulse period, a last pulse period and an intermediate bias period therebetween, instead of the recording pulse waveform including the pulse train, at the time of the high speed recording (see Japanese Patent Applications Laid-open under No. 2003-77128 and No. 2003-85753, for example).
Even when the above-mentioned recording pulse waveform is used, there occurs a problem of a thermal interference in such a case that a space period between the continuous recording marks is short or the laser power in the space period is large. The thermal interference is that heat given to the recording surface of the optical disc by irradiation of the recording laser at the time of recording a certain recording mark gives an adverse effect on the recording of the next recording mark as residual heat. If the thermal interference occurs, a starting position and a length of the recording mark formed after the short space period are changed, and a recording mark of an appropriate shape cannot be formed.